Cold cathode fluorescent lamp in the HT46R14 Converter
Liquid crystal display (LCD) panel has a wide range of applications, from small portable electronic equipment, to large fixed equipment, applications include digital cameras, notebook computers, personal data assistant machine, desktop computers and industrial equipment products. LCD panel itself is not luminous. Therefore, the general need for backlight system to provide light in the background. Typically, the most common light-emitting device is called a CCFL fluorescent tube.
CCFL (Cold Cathode Fluorescence Lamp) or cold cathode fluorescent lamp is determined by the exchange of a DC converter to drive, and it is usually to provide a wide range of DC voltage input and converts it into an exchange of high-frequency high-voltage output to drive tube. Many applications also need effective lighting to achieve the brightness control of light emitted by the surrounding environment with the coordination of lighting conditions, thereby increasing the lamp and battery life.
Figure 1 CCFL usually from a DC to AC converter drive
In this application example, HT46R14 the mcu will be used as a control CCFL backlight the main components. The characteristics of the MCU through the basic mode of operation and circuit control methods.
Some characteristics of CCFLs must pay attention to:
Figure 2 CCFLs of the characteristics of
Among them, the start or the discharge voltage (Discharge Voltage) is activated or lit lamp (CCFT) the minimum voltage required; lamp voltage (Tube Voltage) or voltage (Operating Voltage) is the lamp was lit after the voltage; lamp current (Tube Current) is the lamp was lit after the current; CCFL lamp brightness directly proportional to the current; frequency drive Driver Frequency) commonly used in AC drive frequency of 30kHz to 70kHz.
The basic mode of operation of CCFL
In order to open or CCFL light tube, the circuit to provide an instant impact, or start voltage, the size of the operation voltage is usually 1.5 or 2 times. By the normal state after the operation, CCFL lamp voltage of 400V to 800Vrms general in the voltage range in accordance with the specific type of CCFL.
Figure 3 CCFL operation
Figure 3, the discharge tube at both ends of the process began in about the emergence of a high-pressure 1000Vrms. This voltage is called voltage or discharge voltage start.
When current flows through the lamp, the lamp impedance decreases, while the rapid decline in voltage lamp. When the current reaches a specific value, the lamp voltage drop stop and CCFL to reach a stable voltage, the voltage at this time called the lamp voltage, operating voltage.
After the restart, it is necessary to keep current. Lamp current directly proportional to the brightness of CCFL, CCFL current increases will be able to improve the brightness, however, the use of excessive current may damage the electrode and result in shortened lamp life. The general operation of CCFL current range between 2-7mA. CCFLs are usually driven by the AC, the typical frequency of exchange of 30-70kHz.
PWM brightness control
Lamp brightness control method is usually to control the PWM duty cycle. This method is intermittently open and close the input signal to the converter, by adjusting the PWM duty cycle to control the lamp brightness. This method can achieve a greater range of brightness control. We can use the duty cycle control method and the use of A / D input to detect the current CCFL tube to control a wide range of adjustment.
PPG by adjusting the pulse width to control the operation of CCFL voltage. PFD by controlling the on / off functions of a fixed frequency PWM and variable through the use of the PWM duty cycle, the user can control the brightness of the larger. Its frequency in the general 100Hz-200Hz.
Figure 4 PWM brightness control
application circuit examples
Figure 5 applies to the MCU control circuit CCFL
Figure 5 is a direct-driven push-pull structure, which is the simplest one of CCFL driver circuit applications. It consists of a micro-controller, two N-MOS field-effect transistor, and a lamp ballast connected in series capacitance and the composition of a high-frequency transformer. The input circuit can be batteries or power supply voltage line provided, it outputs an alternating current source for driving lamp. Its control method are described below:
HT46R14 microcontroller is an example of the main circuit components, can produce a variable frequency pulse width modulation, or PWM output. Control circuitry directly connected to a high-voltage converter, it is mainly through the use of a pair of N-MOS field-effect transistor for the non-resonant modulation methods to drive CCFLs.
HT46R14 microcontroller includes two comparators, two PPG (Programmable Pulse Generator) output, with two regular PFD function, counters and 8-channel, 9-resolution ADC function. Users can make use of these features have a variable frequency PWM output, and control of activated CCFL lamp current and lamp brightness.
Figure 5 is a CCFL used in the MCU control circuit. PFD output at the same time connected to the comparator input 1 (-) and pin INT1B. Users through software commands to start PFD output, resulting in a 50% duty cycle signal, its frequency can Timer / Counter 0 or Timer / Counter 1 overflow to control the cycle. PFD clock from the two timer / counter. Frequency set by the membership by the Timer / Counter registers in the loading needed to achieve value. This PFD can be a CCFL lamp output frequency to match the frequency, typically 30-70kHz. Users can control the frequency and pulse width PPG cycle drive CCFL.
PPG1 output PPG0 and are connected to the N-MOS field-effect transistor Q1 and Q2, which in the transformer primary-side to form a push-pull structure. PPG1 for PPG0 and alternately open the Q1 and Q2. PPG0 used as a comparator output control signal to start. When the comparator 1 starts, it will decrease the output of a signal edge is beginning to export PPG0 until PPG0 Timer / Counter overflow, so that the output PPG0 stop. PPG1 used as a starting INT1B control signal, the signal triggered by the drop in margin. PPG1 control method with the same PPG0.
Figure 6 PPG output square-wave
PFD output when a change from low to high signal, and potential higher than Vref (Recommended value is 1/2VDD), comparator output a signal edge of a drop in the beginning count PPG0. Similarly, when a high to low PFD output signal change, PPG1 output started counting. PFD user can adjust the output by software PPG timer / counter values to control or PPG output cycle (pulse width). PPG output pulse width should be less than one PFD for each high-or low-potential-bit width. By regulating the cycle, the user greater control of access to the lamp voltage. PPG output square wave as shown in Figure 6.
In order to control the lamp brightness, resistance R1 in series with the CCFL, and its feet in the MCU to detect the CCFL current AN0 part, be used as a feedback pressure components.
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